The present invention relates to an optical cable which comprises a fiber core, a fiber cladding, a protective polyamide layer firmly adhering thereto, and an external layer.
Polymeric optical fibers (referred to below by the abbreviated term POFs) are used in the telecommunications sector as optical transmission components which provide resistance to failure and simplicity of operation wherever the distance between the transmitter and receiver is only a few meters to a maximum of about 150 m. POFs are also of increasing importance in the sectors of traffic engineering/vehicle construction (data transmission and signal transmission in motor vehicles, aircraft, ships, etc.), lighting (variable traffic signs), automation technology (machine control), and sensor technology (see, for example, Draht 46 (1995) 4, pp. 187-190).
A POF serving for data transmission or signal transmission is composed of a fiber core often manufactured from polymethyl methacrylate (PMMA; refractive index xcex7PMMA=1.49), and of a single- or multilayer fiber cladding concentrically sheathing the fiber core. The cladding material mainly used is fluorinated polymers, the refractive index of which is in the range from 1.35 to 1.42. The optical attenuation of such a POF is typically from 130 to 150 db/km (xcex=650 nm), and the minimum bending radius is from about 5 to 10 mm.
In order to protect the sensitive POF from mechanical, thermal, and chemical effects it is provided with a polymer sheath functioning as protective covering, and the sheath may also, where appropriate, have a multilayer structure (WO 99/12063). The polymer sheath applied by means of an extruder can, depending on the application or application sector, be composed of polyethylene (PE), polyvinyl chloride (PVC), ethylene-vinyl acetate (EVA), or polyamide (PA), for example.
In the automobile construction sector, polyamides are used as protective covering material, since they comply with the requirements prevailing in that field in terms of mechanical strength (primarily tensile strength and crush resistance), maximum service temperature, and chemicals resistance. However, the poor adhesion of the protective polyamide covering on a POF whose fiber material is composed of a fluorinated polymer poses problems. The weak adhesion of the protective covering is particularly disadvantageous when the optical cable (POF plus protective covering) has been laid in an environment exposed to large temperature variations, e.g. the passenger compartment of a motor vehicle, and the POF moves relative to the protective covering because its thermal expansion properties are different and the adhesion of the polyamide to the fluorinated polymer is poor. An example of a consequence of this is that the distance of the end of the POF from the transmitter and receiver (light-emitting diode/PIN diode) sometimes becomes so great that the intensity losses arising are unacceptably high and in certain cases lead to failure of the data-transmission path. In addition, there is the danger of damage to the transmitter or receiver if there is excessive movement of the POF out of the protective covering.
To suppress this effect, termed xe2x80x9cpistoningxe2x80x9d of the POF, the plugs, couplers, or holders used exert large clamping or crimping forces on the protective covering and thus increase the friction between protective covering and POF. However, the resultant deformation of the interface between fiber core and fiber cladding causes increased signal attenuation.
Although removal of the protective sheathing layer in the plug prevents xe2x80x9cpistoningxe2x80x9d, there is the associated danger of damage to the fiber cladding during assembly due to incorrect operation of the stripping tool with its two blades.
The clamping or crimping forces exerted by the plug on the optical cable can also be reduced by using an interlocking anchoring method for the POF in a cone-shaped hole in the plug housing. For example, one proposal uses a hotplate for partial melting of the end of the POF, presses the resultant molten lip into the hole, which narrows toward the inside of the plug, and so anchors the POF firmly within the plug housing. However, the shape of the POF in the region which has melted and therefore deformed sometimes diverges considerably from the cylindrical shape permitting total reflection, and increased intensity losses therefore arise in the plug housing.
DE 199 14 743 A1 and the equivalent WO 00/60382 give a solution for this problem. The latter discloses an optical cable with a POF which has a fiber core and has a single- or multilayer fiber cladding, and also at least one protective covering surrounding the POF, where the fiber cladding or at least its outer layer is composed of a fluorinated polymer, and the protective covering is composed of polyamides or copolyamides with a melting point below 220xc2x0 C. The protective covering requires no assistance to adhere to the fiber cladding, because the carboxy end group concentration of the polyamide is not more than 15 xcexceq/g and the amino end group concentration is in the range from 50 to 300 xcexceq/g. These polyamides used in WO 00/60382 are of low viscosity in order that they can be extruded onto the fiber cladding at minimum melt temperature. The maximum extrusion temperature is therefore only from about 185 to 200xc2x0 C.
WO 00/60382 mentions the possibility of admixing fillers, such as carbon black, with the protective covering material, or forming the protective covering from two or more layers, but no information of a more specific nature is given.
However, the optical cables disclosed in WO 00/60382 have a number of disadvantages:
adequate adhesion is not always achieved at the extrusion temperatures given;
extrusion of a low-viscosity polyamide melt does not give sufficient melt pressure to achieve the required adhesion under tension;
the flame retardancy demanded by the market cannot readily be achieved in the single-layer sheath as proposed, since the flame retardants usually used impair adhesion to the fiber cladding and, furthermore, the optical attenuation is affected by the migration of the flame retardant or by the mechanical action of particles of the flame retardant on the fiber sheath.
It is therefore an object of the present invention to provide a flame-retardant POF which has excellent adhesion of the protective covering to the fiber cladding, and which moreover has a uniform thickness of protective covering.
This and other objects have been achieved by the present invention the first embodiment of which includes an optical cable, comprising:
a polymer optical conductor comprising a fiber core;
a single-layer or multi-layer fiber cladding;
an inner external layer which adheres to said fiber cladding with a peel force of at least 50 N and comprises a first molding composition which comprises a first polyamide; and
an outer external layer which adheres to said inner external layer with a peel force of not more than 30 N;
wherein said first polyamide is selected from the group consisting of a) PA 11, b) PA 12, c) PA 1012, d) PA 1212, e) a copolyamide of at least two of PA 11, PA 12, PA 1012 and PA 1212, said copolyamide containing not more than 30 mol % of a comonomer, and f) mixtures thereof;
wherein said first polyamide contains at least 50 xcexceq/g of amino end groups;
wherein said first molding composition has a zero-shear viscosity of from 400 to 6000 Pas, measured according to ASTM D4440 at 220xc2x0 C.; and
wherein said outer external layer comprises a second molding composition which comprises the following i)-iii):
i) from 20 to 95% by weight of a second polyamide selected from the group consisting of a) PA 11, b) PA 12, c) PA 1012, d) PA 1212, e) a copolyamide of at least two of PA 11, PA 12, PA 1012 and PA 1212, said copolyamide containing not more than 30 mol % of a comonomer, f) a polyetheramide of at least one of a)-e), and g) mixtures thereof,
ii) from 5 to 45% by weight of a flame retardant, and
iii) from 0 to 60% by weight of an impact modifier,
wherein an amount of i), ii) and iii) is based on a total amount of i)+ii)+iii).